A thermocouple is typically embedded in an insulator, such as magnesium oxide, and encased along with the insulator in a metal sheath. For temperature measurement in the range of 600° C. to 1100° C. type K, or, more preferably for the higher temperature range, type E or N thermocouples are typically used as they are more cost effective than type B, R and S thermocouples which are made of more exotic materials capable of operating in still higher temperature regions.
While the sheath of the thermocouple may or may not be directly exposed to process gases, it nonetheless is desirable to provide the sheath with an oxygen resistant barrier so as to minimize oxygen diffusing into the cavity of the sheath. If oxygen is allowed to diffuse, oxidation of the thermocouples wires may occur. This may lead to measurement accuracy degradation as the electrical properties of the wires are changed and ultimately to breakage as oxidation reduces mechanical strength. These typical problems predominately define thermocouple service life in hot gas service.
A known method of ameliorating the degradation of thermocouple wires is through material selection of the sheath. While steel alloys are totally unsuitable across the hot gas temperature range, more expensive nickel alloys are only partially suitable at the upper end of the temperature range. More exotic alloys, such as those containing platinum or rhodium, while eminently suitable, are significantly more expensive than nickel alloys. Therefore despite the disadvantages of nickel alloys, nickel alloys typically containing higher than 74% nickel, greater than 15% chromium and between 4 to 5% aluminum, are commonly used in sheaths for hot gas service as the service life advantage of alternative metals does not warrant their higher costs.
As an alternative to manufacturing the entire sheath out of suitable material it is known to apply coatings to sheaths made of less expensive materials. For example U.S. Pat. No. 5,161,297 describes a method of gold coating a thermocouple sheath to enable operation above 540° C. Such a coating has however low abrasion resistance and the method of application significantly increases manufacturing complexity. U.S. Pat. No. 2,571,700 describes another coating method, with similar disadvantage, that involves coating the thermocouple wires directly with chromic oxide.
Another method of forming oxide coatings on the sheath includes the use of controlled oxidation in an oven with gas composition control. The control enables selective oxidisation and reduction of coating layers of the sheath, which can provide a desirable barrier layer. In such as method the oven environment may comprise, for example, pure oxygen, steam/hydrogen or CO2/CO atmospheres in varying ratios, varied during the oxidation process. Maintaining oven gas composition control is however complexity and expensive.
Thermocouple sheaths are quite small and fragile and so are not suitable for more aggressive manufacturing process that may damage the sheath. For example Patent WO 00/11230 that describes an oxide coating forming process for nickel alloy is unsuitable as it includes the unsuitable steps of hot roll and cold roll, which could damage sheaths. Further, as grain growth is promoted by heat treatment steps and the larger the grain size the less the resistance for diffusion should protective oxide layers be damaged, the multiple heat treatment steps of the described process may provide further disadvantages.
There exists therefore a need to provide a practical, alternative means of prolonging the life of thermocouples in hot gas turbine service.